In a message dated 98-11-10 21:53:16 EST, you write:
>
>Hardy and Ken--
>Ok! the new (old stock) head is about to go off to the grinder for
>modifications. The question arises, what should the target compression
>ratio be??
>Kastner's Competition Manual says limit it to 11.7 to 1. What do you
>suggest? Kastner also advised against using "pop-ups" but some, I am
>told, have made them work. These pistons are proper flat-tops. The old
>head--that which kept eating head gaskets--had been milled by .150 thou.
>With 89mm pistons that probably made for some serious compression which is
>why the gaskets kept going away. I would like to think it was just metal
>fatigue--something I, now approaching my 58th year, can well appreciate.
>In any event (and I would like to get back to finishing some of them) your
>thoughts (and those of all FOT'ers) would be appreciated.
>
Opinions...they're just like, well you know, and everybody's got one, here's
mine...just remember, you get what you pay for.
<Personal Opinion Mode On>
I have seen several different ways of obtaining the highest amount of
"squeeze" you can get from the TR motor. The limiting factor seems to be the
thickness of the combustion chamber roof over the squish area. 150 thou will
rarely cause a problem, but as you try to get up past 180 or so, there's that
"artistic creation" problem. Once you crack the head open up there, you have
just created an expensive labor enhanced work of automotive "art".
Some people have cut the blocks down and flycut the head for the extended
piston in the hopes that the stepped areas will add some strength, but this is
a lot of work and you run into valve shrouding problems cause you can't grind
out the edge of the sleeve anymore. And the chamber roof is still the same
thickness.
Legends abound of "angle" milling the head so that you leave as much metal
over the squish area as possible. I have met people who swear that they have
done it to the TR heads, but I've never had one in my hand. The theory sounds
good to me, but you have to remember to parallel cut the area under each head
nut to match, and bore the stud holes straight again.
Probably the cheapest way to get the compression you need, is to use pop up
pistons, but you have to be careful here. Back in the "olden daze", there was
a major piston manufacturing company that sold lots of pop up pistons. Most
all of the English cars pistons I have ever seen from this company had exactly
the same compression inducing pop up on it, namely a "bar" across the top of
the piston. This did indeed raise compression, but in some motors killed flame
front propagation and created major hotspots in others. It was not uncommon to
see the center melted out of one of the pistons. This is not the way to go
with the TR motor. I have used and noted several TR pistons that use a "flat
top" pop up. That is the top of the piston is still flat and the pop up is as
large as you can fit into the combustion chamber. Basically the pop up is just
an extended top of the piston on the spark plug side (check your side
clearance here and allow for a little piston rock) with the squish side
dropping back to the stock crown height (be sure to angle this side well to
allow for the squish area to do its job)
If you cut the head less than 150, it should last longer with these pistons to
get the compression up there. You do want to cut the head to cut off the valve
shrouding and to keep the piston weight as low as possible. I know that
Venolia Pistons can build this design to most any height.
It is a little hard to say what number you should shoot for, but I think 11.7
is a little light. I would be surprised that your previous problem of blowing
gaskets is due to your compression with your setup. Are you using factory
style steel shim gaskets? Are you using the lead solder trick? Are you
grinding the gaskets too far for the 89mm piston size? If you are using the
steel shims, I'm not sure you can cut them far enough for the 89's as these
were originally made for the 83 mm bores.
<Personal Opinion Mode Off>
Compression?...hey, it's all relative...
Nick in Nor Cal
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